This invention relates generally to the field of precision machining, and more specifically to an articulated coordinate measuring machine (ACMM), system, and method for providing independent, real-time position feedback control during precision machining.
Traditionally, the position of a movable machine member (e.g. tool holder or end effector) is determined indirectly by sensing motion at a large distance away from the actual point of operation. For example, the linear motion of a slide for a worktable is derived indirectly from rotational motion of a motor and lead screw combination by using a angle encoder or resolver. Another example is the use of a linear scale located adjacent to a guideway. These indirect methods have limited accuracy due to the well-known Abbe-offset error effect (e.g. comparator error). The accuracy can also be degraded by (1) thermal distortion effects induced by operation of the machine itself (e.g. motor heat from driving a tool under load, and spindle heating due to friction), or by an uncontrolled thermal environment; and (2) by progressive wear or aging of drive gears, guideways, etc. These problems become even more severe as the number of degrees of freedom (DOF) increases, such as a serially-linked 6-axis robotic arm manipulator, or a parallel-linked 6-axis Hexapod or Rotopod machine.
In-process inspection and Statistical Process Control requirements have forced the need to periodically re-certify the manufacturing process. Traceability of the machining process, and quick performance evaluation within the required workspace, can be critically important for small lot size and one-of-a-kind fabrication. The American National Standard ASME B5.54 provides rules for determining the three-dimensional positional performance of computer numerically controlled (CNC) systems. Satisfying the specifications of ASME B5.54 usually requires a multitude of expensive and sensitive equipment, applied by trained experts.
For straightness evaluation of a single machine tool axis, a laser interferometer or a series of calibrated, multiple-length gauge blocks can be used. For two-dimensional positional performance, a precision variable-length ball-bar is recommended. For three-dimensional positional performance (e.g. volumetric), the length of a fixed ball-bar should be measured in 20 different locations along the edges, face diagonals, and body diagonals of a cubical work zone. For non-cubical work zones, the number of positions can increase to 30-35 positions. A need exists, therefore, for a rapid, yet accurate multi-dimensional metrology system for calibrating precision machine tools.
Despite the need for increased accuracy and speed, such a metrology tool should remain a lightweight and portable unit. This would allow a single coordinate measuring machine (CMM) to periodically check and certify an entire shop floor having a multitude of equipment.
Statistical process control (SPC) has been successfully used for quality control of high volume manufacturing. However, as product diversity increases, and lot size decreases, the desire to minimize or eliminate independent product inspections has grown. This has created a new requirement for total process control (TPC), where all of the factors influencing the production process are detected, analyzed, and controlled, preferably in real-time. For this reason, an independent, real-time position feedback control system is needed to build Total Process Control (TPC) into the manufacturing process.
As explained above, real-time position measurement systems used on conventional precision machines suffer from a number of errors (e.g. comparator offset, thermal distortion, and wear of sliding surfaces). An ideal metrology system would directly measure the precise location of the actual cutting point of a machine tool, spray head, etc. during operation.
Laser trackers can provide non-contact, real-time measurement in three dimensions of a retroreflector sphere or cube mounted to a surface. These devices utilize a motorized, gimbal-mounted laser interferometer, which tracks the moving retroreflector (such as the SMX-4500 Laser Tracker manufactured by Spatial Metrix Corp., or a SMART310e Laser Tracker manufactured by Leica, Inc.). However, a laser retroreflector can not be practically mounted to an actual spinning drill bit or cutting tool. More importantly, if an object obstructs the laser beam, tracking can be lost. Operation must be paused to allow re-acquisition of the reflecting target mirror. Generation of cutting chips or small particles during machining operations and large volumes of cutting cooling fluids, can obstruct the laser beam and prevent useful application for real-time position control. End effectors mounted on the end of robotic arms, such as paint sprayers, thermal spray heads, plasma spray heads, sand blasters, grinders, etc. can also produce large volumes of particulates or dust that obscure laser beams, 3-D vision systems, or other non-contact sensors. 3-D laser tracking devices are generally very expensive, in the range of $ 125-250 K.
A need exists, therefore, for a position measurement device that remains in solid contact with the moving member, ideally as close as possible to the cutting tool or point of operation. If the probe tip is physically attached to the moving member, then cutting chips, particulates, and fluids should not interfere with the ACMM""s operation. Ideally, the apparatus would not intrude on the work zone. Also, the device should have a low inertia, so as not to interfere with the rapid motion of the moving member. Such a device should be lightweight, low cost, have low vibrations, have a large range of motion, and a high accuracy. The data collection system of the system should be capable of processing position measurements at a sufficiently high data rate, commensurate with providing real-time feedback to a rapidly moving machine member. The accuracy of such a system should be better than 10 microns, preferably better than 3 microns. Such a system should be easily mounted on, or near, existing machines with minimal structural modifications.
If the probe tip of the ACMM is physically mounted with a pivoting joint to the moving member of the machine tool, the ACMM""s probe could be automatically guided by the moving member through all extremes of the workspace. This could make the application of the ACMM simple and automatic; requiring little skill or training. Existing feedback systems, i.e. resolvers, encoders, linear scales, etc. could be used for the servo controls velocity feedback loop, while using the independent position feedback of a pivotally mounted CMM for the displacement feedback loop.
Gantry or bridge-style fixed 3-axis orthogonal (e.g. Cartesian) CMM""s provide outstanding accuracy (better than 0.0001 inches), but are typically very expensive and are not portable.
Multi-axis, portable CMM""s (ACMM""s) are commercially available from Romer, Inc. of Carlsbad, Calif.; and by Faro Technologies, Inc. of Lake Mary, Fla. However, their accuracy is limited to about 0.001-0.005 inches. These portable ACMM""s have six degrees-of-freedom (one rotation axis and one swivel axis at each of the three joints, linked by two support arms). Six degrees-of-freedom (DOF""s) provides the ability to easily position the probe tip underneath and behind complex shapes, without having to reposition the base. The use of rotary joints also minimizes errors due to torques and bending moments. Precision rotary transducers (e.g. angle encoders) are mounted at each of the six joints. Their data are used to calculate the probe""s position in three-dimensional Cartesian space. The measuring volume is generally spherical, with the radius equal to the maximum reach of the linked arms, typically a 3-6 foot radius. The tubular support arms are typically made of a lightweight and stiff material, such as an aluminum alloy, or a carbon fiber composite.
A need exists, therefore, for a low-cost, portable ACMM that has sufficient accuracy for providing in-process inspection of part features while the part is still mounted on the machine (during pauses in the machining cycle). This would eliminate the need to remove the part and transport it to a fixed, large gantry or bridge-style CMM inspection station. Use of an independent, in-process inspection tool can save time, and eliminate potential errors in re-positioning the part when machining starts-up again.
Some machine tools, such as horizontal and vertical lathes, rotate the workpiece during machining. In this case, because the workpiece geometry is axisymmetric, only 2 degrees-of-freedom are required (e.g. travel down the centerline, Z-axis, and radial extension, R-axis). Consequently, a highly accurate ACMM, mounted to, for example, the tail stock frame of a horizontal lathe, and would only need two independent axes of revolution to provide complete measurement for a lathe-type machine tool. A 2 DOF ACMM could also be used for measuring the contour of a part in a flat, 2-D plane (e.g. X-Y plane). An ACMM with only 1 DOF could be used for performance evaluation and calibration of gantry-style 3-axis orthogonal CMM""s.
Electro-Discharge Machining (EDM) of metal parts involves passing a high current through a wire or sinker electrode, and spark-eroding the workpiece. Due to the high voltages involved, it would be useful if a highly accurate ACMM could withstand high voltages without damage.
Despite the need for increased accuracy, a highly accurate ACMM should remain a lightweight and portable tool. This allows a single metrology tool to periodically check and certify an entire shop floor having a multitude of equipment. The tool should be easily mounted on a working surface on, or near, existing machines. The tool should be capable of hands-off operation (e.g. unattended), after initial setup.
Many approaches can be used to improve the accuracy of ACMM""s to better than 0.001 inches. One method would be to reduce the number of serially linked joints (e.g. from six down to three) because the total position error builds upon the individual position errors for each joint linked in series.
Another method to improve the accuracy of ACMM""s would be to increase the accuracy of the angle encoder(s). One approach would be to wrap a linear encoder tape with a fine line pitch (e.g. fine gradation of marks) around the outside circumference of a circular encoder wheel. As the diameter of the wheel increases, so does the circumference. The larger circumference generates a proportionally larger number of counts (e.g. count rate) sensed by a read head for the same angle of rotation, as compared to a wheel having a smaller diameter that is wrapped with an encoder tape having the same line pitch. Likewise, for the same number of line counts, a wheel having a larger diameter will rotate a smaller angle than a wheel having a smaller diameter. Consequently, the angular accuracy can be increased essentially without limit by increasing the diameter of a wheel that has a linear encoder tape wrapped around the wheel""s circumference.
Conventional ACMM""s have not adopted this approach because the increased weight of the larger diameter encoder wheels reduces the ease of portability, while the increased size could interfere with physical access behind surfaces and inside of small work volumes. Also, it wasn""t until recently that flexible, highly accurate optical or inductive encoder tapes became commercially available in a configuration suitable for wrapping around the circumference of a wheel, at an affordable cost.
Accuracy of the ACMM could also be increased by (1) using materials with a low thermal expansion coefficient, (2) requiring very tight machining tolerances, (3) using high precision ball or roller bearings (e.g. ABEC grade 7-9 ball bearings), and (4) using highly accurate angle encoders (e.g. increasing from 81,000 counts per revolution to 230 million counts per revolution). However, these changes generally increase the overall cost of the ACMM.
A need exists, therefore, to reduce the costs of ACMM""s, while enhancing accuracy. This can be achieved, in part, by reducing the number of DOF""s (e.g. from 6 to 3), which eliminates excess bearings, angle encoders, machining, etc. Also, use of large diameter encoder wheels and wrapped encoder tapes can reduce the costs, as compared to more expensive, commercially available compact laser angle encoders (e.g. Canon K-1 angle encoder).
Reducing the number of DOF""s from six to three could eliminate the need to use two hands to support the serially linked arms. This could also eliminate the need for counterbalancing the arms with springs or weights.
Use of a highly accurate articulated coordinate measuring machine (ACMM) mounted on a working surface, on or near, the machine tool or robotic arm, and having a probe tip pivotally-mounted to the movable machine member, could provide independent, real-time position feedback control needed to build Total Process Control (TPC) into the manufacturing process.
Other applications of using a highly accurate articulated coordinate measuring machine include 3-D digitizing/tracing of surfaces and solid objects; 3-D spatial interfacing with a computer (e.g. a 3-D mouse/joystick); 3-D sculpting via a master-slave arrangement; and remote surgery or micro-surgery via a master-slave arrangement.